Chain saws

ABSTRACT

A chain saw includes side plates defining a cutter bar and a sprocket wheel freely rotatably mounted at the nose end thereof to guide a cutting chain. The axial spacing between the sprocket wheel and the inner surfaces of the side plates is greater at the root portions of the sprocket wheel that at a location remote from the root portions to prevent rubbing which could otherwise produce heat checking at the root portions of the sprocket teeth. In accomplishing this, the root portions can be recessed away from the side plate inner surfaces and/or the side plate inner surfaces can be recessed away from the root portions. The recesses define reservoirs for receiving lubricating medium to maximize lubrication capacity.

United States Patent n91 Ehlen et al.

[ CHAIN SAWS [75] Inventors: Jack W. Ehlen. Torrance; Jay R.

Bailey, Chatsworth. both of Calif.

[73] Assignee: McCulloch Corporation. Los

Angeles. Calif.

[22] Filed: June 27, I974 [2l] App]. No.: 483,820

[52] US. Cl 30/387; 83/824 [51] Int. Cl B271) 17/04 [58] Field Of Search 30/381, 382, 383, 384, 30/385, 386. 387, 3171;83/814, 821, 830, 820. 816. 824

[56] I References Cited UNlTED STATES PATENTS 2.693.206 11/1954 Anttoncn .4 30/384 X 3.3341170 8/1967 MCI'Z v 83/824 35893115 6/1971 LOClll i 83/824 3.762.047 10/1973 Scott-Jackson. .4 83/820 X 3.810.408 5/1974 Somckawa 30/387 [451 Apr. 22, 1975 Primary Examiner-AI Lawrence Smith Assistant Examiner-J. C. Peters Attorney. Agena or FirmBurns, Doane. Swecker & Mathis [57] ABSTRACT A chain saw includes side plates defining a cutter bar and a sprocket wheel freely rotatably mounted at the nose end thereof to guide a cutting chain. The axial spacing between the sprocket wheel and the inner surfaces of the side plates is greater at the root portions of the sprocket wheel that at a location remote from the root portions to prevent rubbing which could otherwise produce heat checking at the root portions of the sprocket teeth. In accomplishing this. the root portions can be recessed away from the side plate inner surfaces and/or the side plate inner surfaces can be recessed away from the root portions. The recesses define reservoirs for receiving lubricating medium to maximize lubrication capacity.

11 Claims, 7 Drawing Figures suanaugz FIG] PATENTEDAPR22|975 CHAIN SAWS BACKGROUND AND OBJECTS OF THE INVENTION The present invention relates to manually operable chain saws and, in particular, to chain saw cutter bars having a sprocket wheel assembly at the nose end thereof.

Presently utilized chain saws commonly include a cutter bar projecting from an engine housing, as shown for example in U.S. Ehlen et al. Pat. Nos. 3,279,508 and 3,538,965, the disclosures of which are herein incorporated by reference. In the arrangements disclosed therein, the cutter bar defines a channel along which an endless cutting chain travels while driven by a drive wheel located adjacent the inner end of the cutter bar. The outer end of the cutter bar presents a curved nose around which the cutting chain turns. A sprocket wheel is freely rotatably mounted at the nose of the cutter bar and is operable to engage the cutting chain and guide it as it travels around the nose. The cutter bar generally includes a pair of side plates which terminate at their outer ends in spaced curved portions which define a recess. The sprocket wheel is rotatably mounted on a bearing assembly within this recess. A lubrication port communicates with the bearing assembly to allow grease or oil to be periodically supplied thereto for lubrication of the bearing and cutting chain.

Although such an arrangement essentially provides long and useful periods of service, an occasional failure in a tooth of the sprocket wheel may occur. The pres ent inventors have recognized the cause of such failure as being due to frictional wear and to adverse metallurgical changes due to heat buildups which occur at the root portions of the sprocket teeth as the result of rubbing between the sprocket wheel and inner faces of the side plates. That is, the high temperatures caused by such rubbing can alter the metallurgical properties of the sprocket teeth by generating areas of brittle, untcmpered martensite at the root portions. This phenomenon is commonly referred to as heat checking. These brittle areas are characterized by low fatigue and impact strength, and are responsible for impact and/or fatigue failures of the sprocket teeth.

It has also been found that the supplying of lubricating medium, such as grease or oil, through the lubrication port provides an effective manner of lubrication. However, only a very limited supply of such medium can be furnished due to the proximate arrangement of parts of the nose of the cutter bar. As a result, the supply of lubricating medium must be replenished more frequently than would be desirable.

It is, therefore, an object of the present invention to eliminate or alleviate problems of the types previously discussed.

It is another object of the invention to provide a novel cutter bar assembly which effectively minimizes the failure of sprocket wheel teeth.

It is a further object of the invention to provide a novel chain saw which essentially eliminates problematic wear and heat buildups at the root portions of the sprocket wheel teeth.

It is an additional object of the invention to provide such a chain saw which reduces the frequency of lubrication replenishment at the nose end of the chain saw.

SUMMARY OF THE PREFERRED EMBODIMENTS OF THE INVENTION 0 root portions. In this fashion, the tendency for rubbing to occur between the sprocket wheel and the side plate inner surfaces occurs at the remote location so as to resist the occurrence of wear and the buildup of heat in the tooth root portions.

The recessed regions serve to define a reservoir for the reception of lubrication medium, thereby enhancing the lubrication receiving capacity of the chain saw.

The greater axial spacing referred to above may be produced by recessing the root portions of the sprocket teeth inwardly, away from the side plate inner surfaces, and/or by recessing the side plate inner surfaces, outwardly, away from the root portions of the sprocket teeth.

THE DRAWING Other objects and advantages of the present invention will become apparent from the subsequent detailed description thereof in connection with the accompanying drawing in which like numerals designate like elements, and in which:

FIG. 1 is a side view of the cutter bar assembly of a chain saw;

FIG. 2 is a cross-sectional view of the cutter bar as sembly taken along line 2-2 of FIG. 1;

FIG. 3 is a cross-sectional view of one preferred form of the cutter bar assembly taken along line 3-3 of FIG.

FIG. 4 is a cross-sectional view similar to FIG. 3 of another preferred form of the cutter bar assembly;

FIG. 5 is a fragmentary view, in perspective, of the outer end of a cutter bar side plate depicted in FIG. 3 showing the inner surface thereof;

FIG. 6 is a perspective illustration of a sprocket wheel in accordance with the preferred cutter bar assembly of FIG. 4; and

FIG. 7 is a cross-sectional view, similar to FIGS. 3 and 4, of yet another preferred form of the cutter bar assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 there is shown a manually portable chain saw 10 comprising an engine housing 12 and a cutter bar assembly 14 mounted thereto and extending outwardly therefrom. The cutter bar assembly 14 includes three elongated flat plates, namely, a center plate 16 and right and lefbhand side plates 18 and 20 (FIG. 2). The side plates 18, 20 are secured to the center plate 16 by welds or other suitable fasteners. These side plates 18, 20 extend outwardly above and below the center plate to form a groove or guide channel 21 (FIG. 2) for the reception of tanged portions 22 of a conventional cutting chain 23 (shown fragmentally in FIG. 1). The cutting chain is driven by a conventional drive wheel 25 located at the inner end of the cutter bar assembly.

Projecting longitudinally along the cutter bar assembly I4 is a mounting slot 24 extending laterally through the bar assembly. The bar assembly may be secured to the engine housing in any suitable fashion, such as by bolts positioned through the slot 24 in the manner described in the afore-mentioned Ehlen et al. patents.

Disposed adjacent the outer end of the plates l6, 18, 20, the cutter bar assembly 14 includes a sprocket wheel assembly 30. This sprocket wheel assembly 30 comprises a sprocketed or toothed wheel 32 and an associated bearing mechanism 34. The center plate 16 terminates short of the outer ends of the side plates 18, such that the side plates are spaced to define a recess receiving the sprocket wheel 32.

The bearing mechanism 34 includes a circular base plate 36 which is affixed to the side plates 18, 20 in any suitable fashion, such as by rivets 38. The bearing plate 36 is disposed within a center bore 40 of the sprocket wheel 32. The bore 40 is larger in diameter than the bearing plate 36 to define an annular track 37 therebetween for the reception of rotary bearing elements 42. The bearing elements 42 support the sprocket wheel 32 for low-friction rotation about the fixed bearing plate 36.

A lubrication port 50 is provided through one of the side plates 20 in communicating relation to the annular track 37. The port is arranged to admit periodically a charge of lubricating medium, such as grease or oil, into the bearing assembly. The sprocket wheel 32 includes an annular hub section 59 and teeth 60 projecting radially therefrom. The teeth 60 have forwardly and rearwardly inclined edges. Between the teeth are pockets 62 in which the tangs 24 of the cutter chain are guided as the chain travels around the nose of the cutter bar assembly 14. The cutter chain drives the sprocket by virtue of engagement of the tangs with the sprocket teeth 60. The sprocket wheel 32 is disposed so as to lift the cutting chain out of engagement with the cutter bar plates as the cutting chain travels around the nose thereof.

It will be apparent that the sprocket wheel 32, as it is being driven by a cutter chain during a cutting operation, may tend to be urged toward engagement with the inside surfaces 64, 66 of the side plates 18, 20. The resulting rubbing action between the sprocket wheel and the side plate inner faces produces wear and heat buildups in the sprocket teeth. As the present inventors have realized, the point of greatest vulnerability of the sprocket wheel to failure as the result of this wear andfor heat build-up lies at the base or root portion 68 of the sprocket teeth 60, where the tooth joins with the hub section 59.

The present invention relates to a dimensioning of the sprocket wheel and the side plate inner surfaces wherein the spacing between the sprocket wheel and the side plate inner surfaces 64, 66 is greater at the tooth root portions than at a location remote from said root portions. In this fashion, the tendency for rubbing to occur between the sprocket wheel and the side plate inner surfaces occurs at the remote location so as to resist the occurrence of wear and the build-up of heat in the tooth root portions.

This spacing arrangement may be provided in numerous ways, with three preferred ways being described subsequently.

In one preferred embodiment of the invention, shown in FIGS. 4 and 6, the root portions 68 of the sprocket wheel teeth are recessed inwardly, away from the side plate inner surfaces 64, 66 to define the region of greater spacing. This recessing takes the form of an annular recess 70 which includes the hub section and extends radially outwardly past the root portions of the teeth. Thus, and as will be apparent from viewing FIG. 4, the spacing between the root portions 68 of the teeth and the side plate inner surfaces 64, 66 is greater than the spacing between peripheral portions 72 of the teeth and the side plate inner surfaces. As a result, tendencies of the sprocket wheel 32 to rubbingly engage the side plate inner surfaces during operation will occur at the outer tooth peripheries 72. The accompanying wear and high heat build-ups are concentrated at the peripheral portions 72 of the teeth which are situated remotely from the root portions 68 and are less susceptible to the type of failure which has heretofore affected the root portions.

Conveniently, the annular recess 70 provides a reservoir for receiving lubricating medium through the opening 50. As a result, a greater amount of lubricating medium can be stored in the chain saw, reducing the required frequency of lubrication replenishment. Moreover, this reservoir is situated opposite the root portions of the teeth 60, enabling the oil or grease con tained therein to draw heat from the root portions of the teeth 50 so as to further resist the formation of problematic heat buildups in these areas. The lubricating medium in the reservoir is located in proximity to the outer periphery of the sprocket wheel, and can thus aid in lubricating such area to reduce the intensity of rubbing which occurs there.

It should be noted that while the preferred embodiment discussed in connection with FIGS. 4 and 6 involves a recessing of all of the hub section 59 of the sprocket wheel 32, it will be apparent that only part of the hub section can be recessed if so desired. It is preferable that a peripheral portion of each tooth be configured to assume at least some of the normal rubbing action since these portions of the teeth are able to expel heat more readily than the inner parts of the sprocket wheel 32.

In another preferred embodiment of the invention shown in FIGS. 3 and 5, the regions of the side plate inner surfaces 64, 66 which are located opposite the root portions of the teeth are recessed outwardly, away from the root portions, to define the zone of greater spacing. This recessing takes the form of an annular recess 76 lying opposite the root portions 68 of the sprocket teeth. As in the case of the embodiment described in connection with FIGS. 4 and 6, any tendencies for the sprocket wheel 32 to rubbingly engage the side plate inner surfaces during operation occur at the outer tooth peripheries 72 disposed opposite peripheral portions 77 of the side plates. Consequently, the wear and heat buildups which would otherwise be imposed on the root portions of the sprocket teeth will be minimized.

It will be apparent that the recesses 76 in the side plates function in the manner previously discussed, ie to define a reservoir for receiving additional lubricating medium for the bearing assembly 34.

In a further preferred embodiment of the invention, shown in FIG. 7, the tooth root portions and the side plate inner surfaces are recessed, at 80, 82 respectively, to define the greater axial spacing at the tooth root portions. The results are similar to those previously discussed, namely the transfer radially outwardly of the zone where rubbing tends to occur between the sprocket wheel 32 and the side plate inner surfaces 64, 66. Also, the recesses 80, 82 tend to serve as a reservoir for receiving additional lubricating medium.

SUMMARY OF MAJOR ADVANTAGES By essentially eliminating problematic wear and heat buildups at the root portions of the sprocket teeth, failure of the teeth, as far as would otherwise be induced by these conditions, is minimized. As a result, the useful life of the sprocket wheel is increased.

Extra quantities of lubricating medium which are received and stored by the recesses located opposite the tooth root portions reduces the required frequency with which the lubrication medium must be replenished.

The establishment of lubrication-carrying recesses opposite the root portions of the teeth facilitates the withdrawal of heat from the root portions of the sprocket teeth.

Although the invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, modifications, substitutions and deletions not specifically described may be made without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

l. [n a chain saw having manually portable frame means, a cutter bar assembly connected to said frame means and arranged to receive an endless cutting chain, and power means mounted on said frame means for operable connection to said cutting chain for driving said cutting chain on said cutter bar assembly, said cutter bar assembly comprising:

first and second side plates for supporting and guiding said cutting chain;

said side plates having inner surfaces that are spaced adjacent the outer ends thereof to define a recess;

bearing means mounted in said recess; and

sprocket wheel means mounted within said recess for axial rotation about said bearing means, said sprocket wheel means including:

a hub section, and

a plurality of sprocket teeth projecting radially outwardly from said hub section and being arranged to guide said cutting chain around said outer ends of said side plates;

said sprocket wheel and said side plate inner surfaces being dimensioned such that the spacing between said sprocket wheel and said side plate inner surfaces is greater at the root portions of said teeth than at a location remote from said root portions such that the tendency for rubbing to occur between said sprocket wheel and said side plate inner surfaces occurs at said remote location so as to minimize the occurrence of wear and adverse heat-induced metallurgical changes in said tooth root portions.

2. Apparatus according to claim 1 wherein said side plate inner surfaces are recessed away from said tooth root portions to define said greater spacing.

3. Apparatus according to claim I wherein said tooth root portions are recessed away from said side plate inner surfaces to define said greater spacing.

4. Apparatus according to claim 1 wherein said tooth root portions and said side plate inner surfaces are recessed away from each other to define said greater spacing.

5. Apparatus according to claim 1 including passage means for conducting a lubricating medium to said bearing means; said regions of greater spacing at said root portions communicating with said bearing means to define a reservoir for receiving lubricating medium.

6. In a chain saw having manually portable frame means, a cutter bar assembly connected to said frame means and arranged to receive an endless cutting chain, and power means mounted on said frame means for operable connection to said cutting chain for driving said cutting chain on said cutter bar assembly, said cutter bar assembly comprising:

first and second side plates for supporting and guiding said cutting chain;

said side plates having inner surfaces that are spaced adjacent the outer ends thereof to define a recess;

bearing means mounted in said recess; and

sprocket wheel means mounted within said recess for axial rotation about said bearing means, said sprocket wheel means including:

a hub section, and

a plurality of sprocket teeth projecting radially outwardly from said hub section and being arranged to guide said cutting chain around said outer ends of said side plates;

said side plate inner surfaces being recessed away from said teeth at the root portions thereof such that the spacing between said sprocket wheel and said side plate inner surfaces is greater at said root portions than at a location remote from said root portions so that the tendency for rubbing to occur between said sprocket wheel and said side plate inner surfaces occurs at said at least one remote location in a manner minimizing the occurrence of wear and adverse heat-induced metallurgical changes in said tooth root portions.

7. Apparatus according to claim 6 including passage means for conducting a lubricating medium to said bearing means; said regions of greater spacing at said root portions communicating with said bearing means to define a reservoir for receiving lubricating medium.

8. In a chain saw having manually portable frame means, a cutter bar assembly connected to said frame means and arranged to receive an endless cutting chain, and power means mounted on said frame means for operable connection to said cutting chain for driving said cutting chain on said cutter bar assembly, said cutter bar assembly comprising:

first and second side plates for supporting and guiding said cutting chain;

said side plates having inner surfaces that are spaced adjacent the outer ends thereof to define a recess;

bearing means mounted in said recess; and

sprocket wheel means mounted within said recess for axial rotation about said bearing means, said sprocket wheel means including:

a hub section, and

a plurality of sprocket teeth projecting radially outwardly from said hub section and being arranged to guide said cutting chain around said outer ends of said side plates;

the root portions of said teeth being recessed away from said side plate inner surfaces such that the spacing between said sprocket wheel and said side plate inner surfaces is greater at said root portions than at a location remote from said root portions so that the tendency for rubbing to occur between said sprocket wheel and said side plate inner surfaces occurs at said remote location in a manner minimizing the occurrence of wear and adverse heat-induced metallurgical changes in said tooth root portions.

9. Apparatus according to claim 8 including passage means for conducting a lubricating medium to said bearing means; said regions of greater spacing at said root portions communicating with said bearing means to define a reservoir for receiving lubricating medium.

10. In a chain saw having manually portable frame means, a cutter bar assembly connected to said frame means and arranged to receive an endless cutting chain, and power means mounted on said frame means for operable connection to said cutting chain for driving said cutting chain on said cutter bar assembly, said cutter bar assembly comprising:

first and second side plates for supporting and guiding said cutting chain;

said side plates having inner surfaces that are spaced adjacent the outer ends thereof to define a recess;

bearing means mounted in said recess; and

sprocket wheel means mounted within said recess for axial rotation about said bearing means, said sprocket wheel means including:

a hub section, and

a plurality of sprocket teeth projecting radially outwardly from said hub section and being arranged to guide said cutting chain around said outer ends of said side plates;

the root portions of said teeth and said side plate inner surfaces being recessed away from each other such that the spacing between said sprocket wheel and said side plate inner surfaces is greater at said root portions than at a location remote from said root portions so that the tendency for rubbing to occur between said sprocket wheel and said side plate inner surfaces occurs at said remote location in a manner minimizing the occurrence of wear and adverse heatinduced metallurgical changes in said tooth root portions.

11. Apparatus according to claim 10 including passage means for conducting a lubricating medium to said bearing means; said regions of greater spacing at said root portions communicating with said bearing means to define a reservoir for receiving lubricating medium. i 

1. In a chain saw having manually portable frame means, a cutter bar assembly connected to said frame means and arranged to receive an endless cutting chain, and power means mounted on said frame means for operable connection to sAid cutting chain for driving said cutting chain on said cutter bar assembly, said cutter bar assembly comprising: first and second side plates for supporting and guiding said cutting chain; said side plates having inner surfaces that are spaced adjacent the outer ends thereof to define a recess; bearing means mounted in said recess; and sprocket wheel means mounted within said recess for axial rotation about said bearing means, said sprocket wheel means including: a hub section, and a plurality of sprocket teeth projecting radially outwardly from said hub section and being arranged to guide said cutting chain around said outer ends of said side plates; said sprocket wheel and said side plate inner surfaces being dimensioned such that the spacing between said sprocket wheel and said side plate inner surfaces is greater at the root portions of said teeth than at a location remote from said root portions such that the tendency for rubbing to occur between said sprocket wheel and said side plate inner surfaces occurs at said remote location so as to minimize the occurrence of wear and adverse heat-induced metallurgical changes in said tooth root portions.
 1. In a chain saw having manually portable frame means, a cutter bar assembly connected to said frame means and arranged to receive an endless cutting chain, and power means mounted on said frame means for operable connection to sAid cutting chain for driving said cutting chain on said cutter bar assembly, said cutter bar assembly comprising: first and second side plates for supporting and guiding said cutting chain; said side plates having inner surfaces that are spaced adjacent the outer ends thereof to define a recess; bearing means mounted in said recess; and sprocket wheel means mounted within said recess for axial rotation about said bearing means, said sprocket wheel means including: a hub section, and a plurality of sprocket teeth projecting radially outwardly from said hub section and being arranged to guide said cutting chain around said outer ends of said side plates; said sprocket wheel and said side plate inner surfaces being dimensioned such that the spacing between said sprocket wheel and said side plate inner surfaces is greater at the root portions of said teeth than at a location remote from said root portions such that the tendency for rubbing to occur between said sprocket wheel and said side plate inner surfaces occurs at said remote location so as to minimize the occurrence of wear and adverse heat-induced metallurgical changes in said tooth root portions.
 2. Apparatus according to claim 1 wherein said side plate inner surfaces are recessed away from said tooth root portions to define said greater spacing.
 3. Apparatus according to claim 1 wherein said tooth root portions are recessed away from said side plate inner surfaces to define said greater spacing.
 4. Apparatus according to claim 1 wherein said tooth root portions and said side plate inner surfaces are recessed away from each other to define said greater spacing.
 5. Apparatus according to claim 1 including passage means for conducting a lubricating medium to said bearing means; said regions of greater spacing at said root portions communicating with said bearing means to define a reservoir for receiving lubricating medium.
 6. In a chain saw having manually portable frame means, a cutter bar assembly connected to said frame means and arranged to receive an endless cutting chain, and power means mounted on said frame means for operable connection to said cutting chain for driving said cutting chain on said cutter bar assembly, said cutter bar assembly comprising: first and second side plates for supporting and guiding said cutting chain; said side plates having inner surfaces that are spaced adjacent the outer ends thereof to define a recess; bearing means mounted in said recess; and sprocket wheel means mounted within said recess for axial rotation about said bearing means, said sprocket wheel means including: a hub section, and a plurality of sprocket teeth projecting radially outwardly from said hub section and being arranged to guide said cutting chain around said outer ends of said side plates; said side plate inner surfaces being recessed away from said teeth at the root portions thereof such that the spacing between said sprocket wheel and said side plate inner surfaces is greater at said root portions than at a location remote from said root portions so that the tendency for rubbing to occur between said sprocket wheel and said side plate inner surfaces occurs at said at least one remote location in a manner minimizing the occurrence of wear and adverse heat-induced metallurgical changes in said tooth root portions.
 7. Apparatus according to claim 6 including passage means for conducting a lubricating medium to said bearing means; said regions of greater spacing at said root portions communicating with said bearing means to define a reservoir for receiving lubricating medium.
 8. In a chain saw having manually portable frame means, a cutter bar assembly connected to said frame means and arranged to receive an endless cutting chain, and power means mounted on said frame means for operable connection to said cutting chain for driving said cutting chain on said cutter bar assembly, said cutter bar assembly comprising: first And second side plates for supporting and guiding said cutting chain; said side plates having inner surfaces that are spaced adjacent the outer ends thereof to define a recess; bearing means mounted in said recess; and sprocket wheel means mounted within said recess for axial rotation about said bearing means, said sprocket wheel means including: a hub section, and a plurality of sprocket teeth projecting radially outwardly from said hub section and being arranged to guide said cutting chain around said outer ends of said side plates; the root portions of said teeth being recessed away from said side plate inner surfaces such that the spacing between said sprocket wheel and said side plate inner surfaces is greater at said root portions than at a location remote from said root portions so that the tendency for rubbing to occur between said sprocket wheel and said side plate inner surfaces occurs at said remote location in a manner minimizing the occurrence of wear and adverse heat-induced metallurgical changes in said tooth root portions.
 9. Apparatus according to claim 8 including passage means for conducting a lubricating medium to said bearing means; said regions of greater spacing at said root portions communicating with said bearing means to define a reservoir for receiving lubricating medium.
 10. In a chain saw having manually portable frame means, a cutter bar assembly connected to said frame means and arranged to receive an endless cutting chain, and power means mounted on said frame means for operable connection to said cutting chain for driving said cutting chain on said cutter bar assembly, said cutter bar assembly comprising: first and second side plates for supporting and guiding said cutting chain; said side plates having inner surfaces that are spaced adjacent the outer ends thereof to define a recess; bearing means mounted in said recess; and sprocket wheel means mounted within said recess for axial rotation about said bearing means, said sprocket wheel means including: a hub section, and a plurality of sprocket teeth projecting radially outwardly from said hub section and being arranged to guide said cutting chain around said outer ends of said side plates; the root portions of said teeth and said side plate inner surfaces being recessed away from each other such that the spacing between said sprocket wheel and said side plate inner surfaces is greater at said root portions than at a location remote from said root portions so that the tendency for rubbing to occur between said sprocket wheel and said side plate inner surfaces occurs at said remote location in a manner minimizing the occurrence of wear and adverse heat-induced metallurgical changes in said tooth root portions. 